Polyoxymethylene (also called polyacetal, and abbreviated as POM) resins have an excellent balance of mechanical properties, chemical resistance, sliding properties and the like, and further, their manufacture is easy, and as a result, they have become widely applied as exemplary engineering plastics centered around electric/electronic parts, automotive parts, and various other mechanical parts and the like.
There are also copolymers of polyoxymethylene polymer, and the copolymer is produced by polymerizing with formaldehyde or cyclic multimers thereof as the main monomer, and a cyclic ether and/or cyclic formal as the comonomer, under the presence of a catalyst.
However, a portion of the molecular terminals are unstable terminal groups in the obtained crude polyoxymethylene copolymer, and therefore it is thermally unstable, and when molding, there is thermal decomposition which generates formaldehyde, which may become an environmental problem. Further, the generated formaldehyde oxidizes during molding and becomes formic acid, and this formic acid can cause problems such as decomposing the polyoxymethylene copolymer, and foaming in the resin molded articles, and the like. Further, formic acid may become a cause of rust in metal parts in contact with the resin molding.
In consideration of the above, when obtaining polyoxymethylene resin pellets where various additives such as a formaldehyde scavenger, antioxidant, processing stabilizer, and heat resistance stabilizer and the like are added to a oxymethylene copolymer, normally, the unstable terminal groups of the crude oxymethylene copolymer are stabilized, and the above mentioned additives are added to the stabilized polyoxymethylene copolymer obtained by this stabilization to obtain the polyoxymethylene resin pellets.
As the method of stabilizing the crude polyoxymethylene copolymer, a method of decomposing and removing the unstable terminal groups and the unstable terminal portions which follow these is known.
For example, hydrolyzing the crude polyoxymethylene copolymer, more specifically, heating the crude polyoxymethylene copolymer to a temperature above its fusion point and treating in the fused state to decompose and remove the unstable terminal groups from the crude polyoxymethylene copolymer, or heating the crude polyoxymethylene copolymer at a temperature of 80° C. or more and holding a heterogeneous system in an insoluble liquid medium and decomposing and removing the unstable terminal groups from the crude polyoxymethylene copolymer, and the like are known (refer to Patent Document 1).
Further, using ammonia, an aliphatic amine such as triethylamine, tri-n-butylamine, triethanolamine, and the like, a quaternary ammonium salt such as tetrabutyl ammonium hydroxide and the like, or a hydroxide, inorganic weak acid salt, or organic acid salt of an alkali metal or alkaline earth metal, and decomposing the unstable terminal groups under its presence is also known (refer to Patent Document 2). Further, it has also been proposed to decompose the unstable terminal groups under the presence of a quaternary phosphonium salt, a PN bond-comprising compound, a heterocyclic quaternary ammonium salt, betaine, betaine derivative, amine oxide, or hydrazinium salt or the like (refer to Patent Documents 2 to 5).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-37445
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-1850
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-31348
Patent Document 7: Japanese Unexamined Patent Application, Publication No. 2007-332227
Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2007-112959